1. Field of the Invention
The present invention relates to an acoustic feedback suppression device particularly for auxiliary ringers in plug-in telephone systems.
2. Discussion of the Related Art
As is known, an additional electromechanical ringer is usually present in plug-in telephone systems. This ringer is always connected to the telephone line even when no telephone sets are connected to the sockets, thereby allowing for detection of any incoming calls and connection of a telephone set.
Unfortunately, the presence of the additional ringer introduces an unpleasant acoustic feedback when a telephone number is dialed with a pulse-dial selector while a telephone set is connected to the line.
With the pulse-dial signaling, the line is in fact suddenly interrupted a number of times corresponding to each individual selected digit. The interruption frequency is normally equal to 10 Hz, with an open/close ratio of 1.5 to 2.0.
Additionally, due to exchange inductance, voltage pulses of random amplitude (overvoltages) occur during the repeated opening of the telephone line. These voltage pulses are the pulses which can consequently activate the additional ringer in a spurious manner, causing the above-mentioned unwanted acoustic feedback.
In order to eliminate this type of problem, it is necessary to limit the voltage across the additional ringer. This limiting action must be performed in AC during pulse-dialling, whereas the limiting circuit must be decoupled from the telephone line when connection to the line occurs in the on-hook condition or during ringing.
Some known solutions are discussed below. A first solution provides for the insertion, by means of an appropriate switch, of a voltage limiter, as more clearly described hereinafter, constituted by a pair of Zener diodes connected back to back, of a DC decoupling capacitor and of an insertion switch.
In the case of an electronic insertion switch there is also the problem that the telephone line does not have a set polarity, and it is thus necessary to overcome this problem as well. A solution to this further problem is given by the insertion of an additional bridge which simplifies the execution of the electronic switch.
The above described solutions are particularly complicated to implement on a single silicon substrate.
The aim of the present invention is to eliminate or substantially reduce the problems described above by providing an acoustic feedback suppression device, particularly for auxiliary ringers in plug-in telephone systems, which allows for the elimination or substantial suppression of the acoustic feedback of the additional ringer during pulse-dialing.
Within the scope of the above aim an object of the present invention is to provide a suppression device which controls the voltage across the additional ringer, limiting any spurious activation pulses.
Another object of the present invention is to provide an acoustic feedback suppression device which is highly reliable, relatively easy to manufacture and at competitive costs with respect to mechanical switches.